Fine carbon fiber, method for producing the same and use thereof

ABSTRACT

A vapor grown fine carbon fiber, each fiber filament of the carbon fiber comprising, in its interior, a hollow space extending along the fiber filament, and having a multi-layer structure, an outer diameter of 2 to 500 nm, and an aspect ratio of 1 to 100, wherein the fiber filament comprises a cut portion on its surface along the hollow space, a production method therefor, and electrically conductive material, a secondary battery and a gas occlusion material using the carbon fiber. The fine carbon fiber of the present invention is excellent in properties such as occlusion of gases such as hydrogen and methane, smoothness, electrical conductivity and thermal conductivity, and also excellent in dispersability, wettability and adhesion with a matrix such as resin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is an application filed pursuant to 35 U.S.C. Section 111(a) withclaiming the benefit of U.S. provisional application Ser. No. 60/407,705filed Sep. 4, 2002 under the provision of 35 U.S.C. 111(b), pursuant to35 U.S.C. Section 119(e)(1).

TECHNICAL FIELD

The present invention relates to fine carbon fiber which exhibitsexcellent adhesion to a matrix formed of, for example, resin, ceramic,or metal, which can be uniformly dispersed in the matrix, and which hasa low aspect ratio (length of fiber/diameter of fiber); and to a methodfor producing the fine carbon fiber.

More particularly, the present invention relates to fine carbon fiberhaving a low aspect ratio and having, on its surface, a functional groupcapable of improving wettability to a matrix, the carbon fiber beingproduced through wet treatment of vapor grown carbon fiber; and to amethod for producing the fine carbon fiber.

The present invention also relates to fine carbon fiber having a lowaspect ratio, which is useful as a filler material for improvingelectrical conductivity and thermal conductivity, as an electronemission material for producing field emission displays (FEDs), as amedium for sorption of hydrogen, methane, or various other gasses, andas a material employed in, for example, transparent electrodes,electromagnetic wave shielding materials, and secondary batteries; andto a method for producing the fine carbon fiber.

The present invention also relates to a battery electrode containing thefine carbon fiber so as to attain improved charge/discharge capacity andexhibit improved strength, the battery electrode being employed as apositive or negative electrode of any of a variety of secondarybatteries such as dry batteries, lead storage batteries, capacitors, andrecently developed Li-ion secondary batteries.

BACKGROUND ART

Carbon fiber is used in a variety of composite materials, by virtue ofits excellent properties such as high strength, high elastic modulus,and high electrical conductivity. In recent years, in conjunction withdevelopments in electronic techniques, carbon fiber has been considereda promising electrically conductive filler for producing electromagneticwave shielding materials or antistatic materials, and has been viewed asa useful antistatic filler which can be incorporated into resin or as apromising filler employed in transparent electrically conductive resin.Also, by virtue of its excellent tribological characteristics and highwear resistance, carbon fiber has been considered as a promisingmaterial which is applicable for use in, for example, electric brushesand variable resistors. In addition, carbon fiber has become of interestas a wiring material for producing devices such as LSIs, since itexhibits high electrical conductivity, high thermal conductivity, andelectromigration resistance.

Conventional carbon fiber produced through carbonization of organicfiber by means of heat treatment in an inert atmosphere, such aspolyacrylonitrile (PAN)-based carbon fiber, pitch-based carbon fiber, orcellulose carbon fiber, has a relatively large diameter; i.e., 5 to 10μm, and exhibits poor electrical conductivity. Therefore, such carbonfiber has been widely employed as a reinforcement material in, forexample, resin or ceramic.

In the 1980's, studies were conducted on a process for producing vaporgrown carbon fiber through thermal decomposition of a gas of, forexample, hydrocarbon in the presence of a transition metal catalyst.Through such a process, vapor grown carbon fiber having a diameter ofabout 0.1 to about 0.2 μm (about 100 to about 200 nm) and an aspectratio of about 10 to about 500 has been produced. A variety of processesfor producing vapor grown carbon fiber are disclosed, including aprocess in which an organic compound such as benzene, serving as a rawmaterial, and an organo-transition metallic compound such as ferrocene,serving as a catalyst, are introduced into a high-temperature reactionfurnace together with a carrier gas, to thereby produce vapor growncarbon fiber on a substrate (Japanese Patent Application Laid-Open(kokai) No. 60-27700); a process in which vapor grown carbon fiber isproduced in a dispersed state (Japanese Patent Application Laid-Open(kokai) No. 60-54998, U.S. Pat. No. 4,572,813); and a process in whichvapor grown carbon fiber is grown on a reaction furnace wall (JapanesePatent No. 2778434).

Since vapor grown carbon fiber is formed of carbon which is readilygraphitized, when the carbon fiber is subjected to heat treatment at2,000° C. or higher, the resultant carbon fiber exhibits excellentcrystallinity and improved electrical conductivity. Therefore, thethus-graphitized carbon fiber is employed as an electrically conductivefiller material in, for example, a resin or an electrode of a secondarybattery.

A characteristic feature of each fiber filament of vapor grown carbonfiber resides in its shape and crystal structure. The fiber filament hasa cylindrical structure including a very thin hollow space in its centerportion, and a plurality of carbon hexagonal network layers grown aroundthe hollow space so as to form annual-ring-like tubes. When vapor growncarbon fiber is subjected to heat treatment at 2,000° C. or higher, thecross section of each fiber filament of the thus-treated carbon fiberassumes a polygonal shape, and in some cases, micropores are formed inthe interior of the fiber filament.

Since vapor grown carbon fiber has a small diameter, the carbon fiberhas a relatively high aspect ratio. Generally, fiber filaments of thecarbon fiber are entangled with one another to form fuzzball-likeagglomerates.

Since vapor grown carbon fiber contains thermally decomposed carbonlayers, the carbon fiber has a smooth surface. When such vapor growncarbon fiber is thermally treated at 2,000° C. or higher in an inertatmosphere, the thus-treated carbon fiber exhibits high crystallinity,and smoothness of its surface is further enhanced. The carbon fiberwhich has undergone heat treatment at high temperature has virtually nofunctional groups on its surface.

Since fiber filaments of vapor grown carbon fiber are entangled with oneanother to form agglomerates like fuzzballs, when the carbon fiber ismixed with a matrix formed of, for example, resin or ceramic, the carbonfiber fails to be uniformly dispersed in the matrix, and thuselectrical, thermal, and mechanical characteristics of interest cannotbe obtained.

When such carbon fiber having a high aspect ratio is mixed with a resinso as to form a composite material, and the surface of the compositematerial is observed under a scanning electron microscope, the surfaceof the composite material is found to be not smooth but “hairy” withpieces of the carbon fiber not covered with resin. When the compositematerial is employed as an antistatic material for producing, forexample, an integrated circuit (IC) tray, due to generation ofmicroscratches at a point at which the tray is in contact with a disk orwafer, or deposition of impurities caused by falling of the carbonfiber, the quality of the disk or wafer is lowered, and the yield of afinal product is reduced.

When carbon fiber exhibits insufficient wettability and affinity to amatrix formed of, for example, resin, adhesion between the carbon fiberand the matrix is lowered. Therefore, mechanical strength of theresultant composite material is lowered, falling of the carbon fiberoccurs, and the quality of the composite material is deteriorated.

In view of the above problems, various attempts have been made to reducethe length of long carbon fiber through grinding, in order to improvedispersibility of the carbon fiber and to obtain a composite material ofsmooth surface in relation to the use as a filler. Conventionally,carbon fiber has been ground through dry grinding by use of, forexample, a ball mill, to thereby form short carbon fiber (JapanesePatent Application Laid-Open (kokai) No. 1-65144, U.S. Pat. No.4,923,637 and Japanese Patent Application Laid-Open (kokai)No.11-322314). However, grinding of carbon fiber through impact grindingby use of, for example, a ball mill or a roll mill involves thefollowing problems. Although entangled fiber filaments of the carbonfiber are fragmented through such grinding, fine carbon fiber fragmentsgenerated through grinding form agglomerates in a mill or the fragmentsare bonded together when grinding reaches a certain degree. Therefore,micronization of the carbon fiber does not proceed further, even ifgrinding is performed for a long period of time. In addition, theresultant carbon fiber fragments have a length as large as about someμm.

DISCLOSURE OF INVENTION

An object of the present invention is to provide fine carbon fiberhaving a diameter of 500 nm or less and an aspect ratio of 100 or less,exhibiting excellent tribological characteristics, electricalconductivity, and thermal conductivity, and exhibiting excellentdispersibility in a matrix formed of, for example, resin, and excellentwettability and adhesion to the matrix.

Generally, in order to improve adhesion between carbon fiber and amatrix, carbon fiber having a reduced diameter is used to increase thecontact area between the carbon fiber and the matrix, and in order toimprove wettability or adhesion of carbon fiber to a resin serving as amatrix, the carbon fiber is subjected to oxidation, or a functionalgroup is introduced on the surface of the carbon fiber. With a view tothe above-mentioned problems, the present inventors have made intensivestudies and have found that when fine carbon fiber having a high aspectratio in which fiber filaments are entangled with one another issubjected to wet grinding, agglomerates formed of the filaments can befragmented within a short period of time, and fine carbon fiber havingan aspect ratio of interest can be produced; that a functional group ispresent on the surface of cut portion (the point of rupture) of thethus-ground fine carbon fiber, and presence of the functional groupimproves adhesion between the carbon fiber and a matrix formed of, forexample, resin; and that the amount and type of the functional grouppresent on the surface of the carbon fiber can be regulated by varyingthe type of a surfactant or an organic solvent employed when a slurry isformed from the fine carbon fiber.

According to the present invention, a fine carbon fiber having a lowaspect ratio, which can be uniformly dispersed in a matrix formed of,for example, resin, ceramic, or metal, to thereby improve smoothness ofthe surface of the resultant composite material, which has a functionalgroup on its surface, and which exhibits excellent adhesion to a matrix,can be easily produced through grinding.

Accordingly, the present invention relates to the following fine carbonfiber, production method therefor and use thereof.

1) A vapor grown fine carbon fiber, each fiber filament of the carbonfiber comprising, in its interior, a hollow space extending along thefiber filament, and having a multi-layer structure, an outer diameter of2 to 500 nm, and an aspect ratio of 1 to 100, wherein the fiber filamentcomprises a cut portion on its surface along the hollow space;

2) The fine carbon fiber according to 1) above, wherein the cut portionon the surface of the fiber filament comprises a minute depression;

3) The fine carbon fiber according to 2) above, wherein the minutedepression is in communication with the hollow space in the interior ofthe fiber filament;

4) The fine carbon fiber according to any one of 1) through 3) above,wherein the fiber filament has a functional group on a surface thereof;

5) The fine carbon fiber according to 4) above, wherein the functionalgroup is at least one species selected from the group consisting of ahydroxyl group, a phenolic hydroxyl group, a carboxyl group, an aminogroup, a quinonyl group, and a lactone group;

6) The fine carbon fiber according to any one of 1) through 5) above,wherein the hollow space is partially closed;

7) The fine carbon fiber according to any one of 1) through 6) above,wherein the carbon fiber comprises carbon having an average interlayerdistance d₀₀₂ of (002) carbon layers measured by X-ray diffractionmethod is 0.342 nm or less;

8) The fine carbon fiber according to any one of 1) through 6) above,which further comprises boron or a boron compound;

9) The fine carbon fiber according to 8) above, wherein boron iscontained, in an amount of 0.01 to 5 mass %, in carbon crystalsconstituting the carbon fiber;

10) A fine carbon fiber mixture comprising a fine carbon fiber asrecited in any one of 1) through 9) above in an amount of 5 mass % to 80mass % on the basis of the entirety of the carbon fiber mixture;

11) A method for producing a fine carbon fiber, comprising a step ofwet-grinding vapor grown carbon fiber containing branched vapor growncarbon fiber in the presence of water and/or an organic solvent, eachfiber filament of the carbon fiber comprising, in its interior, a hollowspace extending along the fiber-filament, and having a multi-layerstructure, an outer diameter of 2 to 500 nm, and an aspect ratio of atleast 10;

12) The method for producing a fine carbon fiber according to 11) above,wherein the wet-grinding is performed in the presence of a surfactant;

13) The method for producing a fine carbon fiber according to 11) above,comprising, before the step of wet-grinding, a step of adding or notadding boron or a boron compound to the vapor grown carbon fiber andthen subjecting to heat treatment at a temperature of 2,000° C. to3,500° C.;

14) The method for producing a fine carbon fiber according to 11) above,comprising, after the step of wet-grinding, a step of adding or notadding boron or a boron compound to the wet-ground fine carbon fiber andthen subjecting to heat treatment at a temperature of 2,000° C. to3,500° C.;

15) A fine carbon fiber produced through a production method as recitedin any one of 11) through 14) above;

16) A fine carbon fiber composition comprising a fine carbon fiberproduced through a production method as recited in any one of 1) through9) and 15) above:

17) The fine carbon fiber composition according to 16) above, comprisinga resin;

18) An electrically conductive material comprising a fine carbon fiberas recited in any one of 1) through 9) and 15) above;

19) A secondary battery comprising, as an electrode material, a finecarbon fiber as recited in any one of 1) through 9) and 15) above; and

20) A gas occlusion material comprising a fine carbon fiber as recitedin any one of 1) through 9) and 15) above.

The low-aspect-ratio fine carbon fiber of the present invention is aconventionally unknown low-aspect-ratio fine carbon fiber having minutedepressions and functional groups on the surface, which has been workedout as a result of extensive studies on grinding conditions for vaporgrown fine carbon fiber in order to obtain a carbon fiber exhibitingexcellent adhesion and affinity to resin, and excellent dispersibilityin resin.

The low-aspect-ratio fine carbon fiber of the present invention ispreferably employed as a transparent electrode filler or an occlusionmaterial for gases such as hydrogen and methane, but use of the carbonfiber is not limited thereto, and the carbon fiber may also be employedas an electromagnetic wave shielding material, as a material forimparting electrical conductivity to, for example, a secondary battery,or as a thermally conductive filler. The carbon fiber may also beemployed as a material for imparting electrical conductivity to thesurface of, for example, an OPC drum or a printed circuit board.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 shows the distribution of the lengths of fiber filaments of thefine carbon fiber of Example 1.

FIG. 2 shows the distribution of the lengths of fiber filaments of thefine carbon fiber of Example 2.

FIG. 3 shows the distribution of the lengths of fiber filaments of thefine carbon fiber of Comparative Example 1.

DETAILED DESCRIPTION OF INVENTION

The fine carbon fiber of the present invention will next be described.

The fine carbon fiber of the present invention is a fine carbon fiberproduced through a vapor growth process. Each fiber filament of thecarbon fiber includes a hollow space in its interior, and has amulti-layer structure (annual-ring-like-tube structure), an outerdiameter of 2 to 500 nm, preferably 2 to 200 nm, and an aspect ratio of1 to 100, preferably 3 to 20, wherein the fiber filament has a cutportion on at least a portion of its surface. The cut portion isproduced through, for example, rupture in grinding of carbon fiber, andthe cut portion exhibits high reactivity at its surface by virtue offunctional groups included therein. At the cut portion, carbon atoms atthe edged sites of detects of the basal plane and carbon atoms at theedged sites of boundaries between crystallites are exposed.

The fine carbon fiber of the present invention is produced through thefollowing procedure: vapor grown carbon fiber containing branched vaporgrown carbon fiber is dispersed in, for example, water and/or an organicsolvent, and if desired, a surfactant is added to the resultant mixture;and the mixture is subjected to wet grinding.

The thus-ground carbon fiber is dried to thereby produce fine carbonfiber. Each fiber filament of the carbon fiber has, on its surface alongthe fiber axial direction, merely a cut portion or both a cut portionand a minute depression. In addition, the fiber filament has, on itssurface, a functional group containing oxygen, such as a hydroxyl group,a phenolic hydroxyl group, a carboxyl group, a quinonyl group, or alactone group; an amino group; or a amido group. The fine carbon fiberhaving such a functional group exhibits improved affinity to, forexample, resin, since the functional group contains oxygen or nitrogen.

Each branched fiber filament of vapor grown carbon fiber used in thepresent invention contains a hollow space extending throughout thefilament, though some portions of the hollow space are closed. When thecarbon fiber containing hollow filament with some branches, wherein thebranched portions of the hollow space are in communication with oneanother, is subjected to grinding, ruptures occur in the vicinity ofbranch points of filaments. As a result, minute depressions are formedon the surface of the fiber filament, and each depression comes intocommunication with the hollow space of the fiber filament. Therefore,the resultant carbon fiber exhibits improved wettability and adhesionto, for example, resin.

Moreover, by grinding carbon fiber which contains hollow spaces withsome closed portions, as compared with a carbon fiber with no closedportions, the surface area of filament does not increase too much aftergrinding and the wettability with resin is enhanced. Furthermore, thesurface of ruptured point becomes irregular with small concavities andconvexities to improve adhesion to resin.

The amount and type of the functional group present on the surface ofthe fine carbon fiber of the present invention can be regulated byvarying the type of a surfactant, the type of an organic solvent, anddrying temperature (i.e., temperature at which the functional group isremoved). Thus, wettability and adhesion of the fine carbon fiber toresin can be improved.

Hereinafter, a preferred method for producing the fine carbon fiber ofthe present invention will be described.

The fine carbon fiber of the present invention can be obtained bywet-grinding a vapor grown fine carbon fiber, each fiber filament of thecarbon fiber comprising, in its interior, a hollow space extending alongthe fiber filament, and having a multi-layer structure, an outerdiameter of 2 to 500 nm and an aspect ratio of at least 10, andcontaining branched vapor grown fine carbon fiber, in the presence ofwater and/or an organic solvent.

The carbon fiber employed in the method is generally produced throughthermal decomposition of an organic compound in the presence of anorgano-transition metallic compound.

Examples of the organic compound which may serve as a raw material ofthe carbon fiber include gases such as toluene, benzene, naphthalene,ethylene, acetylene, and ethane, natural gas and carbon monoxide, andmixtures thereof. Of these, aromatic hydrocarbons such as toluene andbenzene are preferred.

An organo-transition metallic compound contains a transition metalhaving a catalytic activity. Transition metal is a metal belonging toGroup IVa, Va, VIIa, VIIa, or VIII (Group 4 to Group 10) of the periodictable. An organo-transition metallic compound such as ferrocene ornickelocene is preferred.

The carbon fiber is produced through the following procedure: theaforementioned organic compound and organo-transition metallic compoundare gasified, the thus-gasified compounds are mixed with a reducing gas(e.g., hydrogen) which has been heated to 500 to 1,300° C., and theresultant mixture gas is fed to a reaction furnace heated to 800 to1,300° C., to thereby allow reaction to proceed.

The material for carbon fiber to be subjected to grinding is preferablytreated at 900 to 1,300° C. in advance to the grinding step, in order toremove an organic substance (e.g., tar) deposited onto the surface ofthe carbon fiber material produced through thermal decomposition.

For wet-grinding, fine carbon fiber is dispersed in water containing asurfactant and/or an organic solvent. The concentration of the finecarbon fiber is within a range of 1 to 30 mass %, preferably 3 to 20mass %, more preferably 5 to 15 mass %. If the concentration is lessthan 1 mass %, grinding efficiency is low. If the concentration exceeds30 mass %, the carbon fiber is not well dispersed in the solvent, anddue to high viscosity of the resultant slurry, fluidity and grindingefficiency are lowered.

Examples of surfactants which may be employed include anionicsurfactants, cationic surfactants, nonionic surfactants, and amphotericsurfactants. Nonionic surfactants, anionic surfactants, and cationicsurfactants are preferred. Specific examples include polyethylene glycolalkyl phenyl ethers such as Triton (Trademark), sulfate salts ofpolyethylene glycol alkyl phenyl ethers, and benzalconium chloride. Theamount of a surfactant added to the carbon fiber is 0.01 to 50 mass %,preferably 0.1 to 30 mass %, on the basis of the entirety of the carbonfiber.

Examples of organic solvents which may be employed include alcohols suchas methanol, ethanol, n-butanol, n-propanol, and n-hexanol; chainhydrocarbons such as n-decane, n-pentane, n-hexane, and n-heptane;aromatic hydrocarbons such as benzene, toluene, and xylene; ketones suchas acetone and methyl ethyl ketone; ethers such as diethyl ether anddibutyl ether; and esters such as ethyl acetate and butyl acetate.

Any known grinding apparatus employing shearing force, compressionforce, or friction force, such as a rotatable cylindrical mill, avibration ball mill, a planetary ball mill, a medium stirring mill, or acolloid mill, may be employed.

A mixture containing the thus-ground carbon fiber is subjected tofiltration and washing, to thereby remove a solvent and a surfactant.Subsequently, the resultant residue is subjected to, for example,hot-air drying, vacuum drying, or freeze drying, to thereby remove thesolvent deposited onto the carbon fiber. By varying the dryingtemperature in removing solvent, among functional groups present on thesurface of the carbon fiber, those of interest can be caused to remainthereon.

In addition, the amount and type of the functional group to beintroduced on the surface of the carbon fiber can be regulated bytreatment employing, for example, hydrochloric acid, nitric acid, orsulfuric acid, or activation treatment employing steam, carbon dioxidegas, or an alkali such as KOH or NaOH.

In the fine carbon fiber thus produced, each filament has a diameter of2 to 500 nm and an aspect ratio of 1 to 100, and filaments, which eachhas a cut portion on its surface along the hollow space inside, accountfor 5 mass % to 80 mass % on the basis of the entirety of the carbonfiber.

Also, in the fine carbon fiber thus produced, the distribution of thelengths of fiber filaments is narrow with the variation in the lengthsbeing small, and the standard deviation (μm) is 2.0 or less, preferably1.0 or less, more preferably 0.5 or less, so that, when the carbon fiberis employed as an electrically or thermally conductive filler, thequality of the composite material can be kept high.

In order to enhance electrical conductivity of the thus-ground and driedfine carbon fiber, before or after grinding and drying, the fine carbonfiber may be subjected to heat treatment at 2,000 to 3,500° C. in aninert atmosphere to increase the graphitization degree of the carbonfiber. In order to further enhance electrical conductivity, the finecarbon fiber may be mixed with a boron compound such as boron carbide(B₄C), boron oxide (B₂O₃), elemental boron, boric acid (H₃BO₃), or aborate, and then subjected to heat treatment at 2,000 to 3,500° C. in aninert atmosphere.

Since thus-graphitized carbon fiber exhibits high crystallinity andenhanced mechanical strength, grinding the graphitized carbon fiber intocarbon fiber having a length of interest requires a large amount ofenergy and a long time in comparison with non-graphitized carbon fiber.

No particular limitation is imposed on the amount of a boron compoundadded to the fine carbon fiber, since the amount varies in accordancewith chemical properties and physical properties of the boron compound.For example, when boron carbide (B₄C) is employed, the amount of boroncarbide is 0.05 to 10 mass %, preferably 0.1 to 5 mass %, on the basisof the entirety of the fine carbon fiber. Through heat treatment in thepresence of the boron compound, the fine carbon fiber exhibits improvedelectrical conductivity, along with improved carbon crystallinity(interlayer distance d₀₀₂). Specifically, the average interlayerdistance d₀₀₂ of (002) carbon layers measured by X-ray diffractionmethod is 0.342 nm or less when neither boron nor boron compound isadded to the carbon fiber. When boron or a boron compound is added tothe carbon fiber, the average interlayer distance (d₀₀₂) is 0.338 nm orless.

Any heat treatment furnace may be employed, so long as the furnace canmaintain a predetermined temperature of at least 2,000° C., preferablyat least 2,300° C. A typically employed furnace, such as an Achesonfurnace, a resistance furnace, or a high-frequency furnace, may beemployed. In some cases, carbon powder or carbon fiber may be heatedthrough direct application of electricity.

Heat treatment is carried out in a non-oxidative atmosphere, preferablyin an atmosphere of one or more rare gasses such as argon, helium, andneon. From the viewpoint of productivity, heat treatment is preferablycarried out within a short period of time. When carbon fiber is heatedover a long period of time, the carbon fiber is sintered to formaggregate, resulting in low production yield. After the center of carbonfiber block is heated to a target temperature, the carbon fiber block isnot necessarily maintained at the temperature for more than one hour.

When carbon fiber is subjected to heat treatment, a portion of thecarbon fiber is sintered to thereby form a sintered aggregate, as in thecase of a typical carbon fiber product. Since the resultant sinteredaggregate of carbon fiber cannot be added to electrodes, etc. oremployed as an electron emission material, the aggregate must besubjected to crushing, to thereby obtain fine carbon fiber suitable foruse as a filler material.

Therefore, the resultant block is subjected to crushing, pulverization,and classification, to thereby obtain fine carbon fiber suitable for useas a filler material. Simultaneously, separation of a non-fibrousproduct is carried out. When the block is insufficiently pulverized, theresultant carbon fiber fails to be mixed with an electrode materialsatisfactorily, and thus the effect of the carbon fiber is not obtained.

In order to obtain fine carbon fiber suitable for use as a filler,firstly, carbon fiber block formed through heat treatment is crushedinto pieces having a size of 2 mm or less, and then pulverized by use ofa pulverization apparatus. Examples of the crushing apparatus which maybe employed include a typical ice crusher and a rotoplex.

Examples of the pulverization apparatus which may be employed includeimpact-type pulverization apparatuses such as a pulverizer and a ballmill; autogeneous grinding apparatuses; and pulverization apparatusessuch as a micro jet. Separation of a non-fibrous product may be carriedout through, for example, air classification.

The fine carbon fiber of the present invention may be incorporated intoa battery electrode, to thereby improve properties such ascharge/discharge capacity and strength of electrode in the resultantbattery. Examples of the battery include batteries which requireimproved electrical conductivity of electrodes and which requireperformance of intercalation, such as a lithium battery, a lead storagebattery, a polymer battery, and a dry battery.

By virtue of its high electrical conductivity, when the fine carbonfiber of the present invention is employed in such a battery, theelectrical conductivity of the resultant battery can be enhanced. Whenthe fine carbon fiber is employed in a lithium battery, thecharge/discharge capacity of the battery can be increased, since thefine carbon fiber exhibits high intercalation performance as a carbonmaterial for a negative electrode.

The amount of the fine carbon fiber incorporated as thus obtained intoan electrode is preferably 0.1 mass % to 20 mass % inclusive. When theincorporation amount exceeds 20 mass %, the packing density of carbon inthe electrode is lowered, thereby lowering the charge/discharge capacityof the resultant battery. In contrast, when the incorporation amount isless than 0.1 mass %, the effect of the fine carbon fiber is low.

When an electrode, for example, a negative electrode of a lithiumbattery containing the fine carbon fiber of the present invention, isformed, the fine carbon fiber and a binder are added to a carbonaceousmaterial such as graphite powder or mesophase carbon micro beads (MCMB),and the resultant mixture is sufficiently kneaded such that the carbonfiber is dispersed in the mixture as uniformly as possible.

The fine carbon fiber of the present invention, as is, as mixed withother carbon fibers, or as a composite mixed with a matrix such asresin, ceramics and metals, may be put to various uses. When employedwith a resin used as a matrix, the composite is adjusted so that theconcentration of the fine carbon fiber of the present invention is 5 to50 mass % based on the amount of the resin component. Examples of resinwhich may be used as matrix in the present invention includethermosetting resins such as phenol resin, epoxy resin, polyurethaneresin, polyimide resin and unsaturated polyester resin; thermoplasticresins such as polyamide resin, polyurethane resin, vinyl chlorideresin, acrylic resin and cellulose resin; and rubbers such as siliconerubber, polyurethane rubber, styrene butadiene rubber and naturalrubber.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will next be described in more detail by way ofExamples, which should not be construed as limiting the inventionthereto.

In order to calculate the amount of branched carbon fiber filament (mass%), a transmission electron micrograph (TEM) of a cross section ofcarbon fiber was used to calculate the ratio of the cross-sectional areaof the branched carbon fiber to the total cross-sectional area of thecarbon fiber, on the assumption that the branched carbon fiber and thecarbon fiber have the same specific gravity.

In order to obtain the amount of boron (mass %), calcium carbonate wasadded to a powdery carbon fiber sample; the resultant mixture was formedinto ash under an oxygen stream; calcium carbonate was added to theresultant ash; the thus-obtained mixture was melted under heating; theresultant molten product was dissolved in water; and the thus-preparedaqueous solution was subjected to quantitative analysis by means of ICPspectroscopy (Inductively coupled plasma atomic emission spectrometrymethod).

EXAMPLE 1

Vapor grown carbon fiber (2 g) having an average diameter of 25 nm, anaverage length of 10,000 nm, and an aspect ratio of 400 and containingbranched vapor grown carbon fiber in an amount of 30 mass %, ethanol (50g), and zirconia beads (diameter of each bead: 1.0 mm) (200 g) wereplaced in an agate-made planetary mill (capacity: 300 cm³), and thecarbon fiber was subjected to grinding treatment for four hours.Subsequently, the thus-ground carbon fiber was dried at 150° C. forthree hours. Thereafter, the resultant vapor grown carbon fiber wasobserved under a scanning electron microscope, and the lengths of fiberfilaments of the carbon fiber were measured. In addition, the carbonfiber sample was subjected to infrared analysis.

As a result, a fine carbon fiber having an average diameter of 25 nm, anaverage length of 250 nm, an aspect ratio of 10 and d₀₀₂ of 0.340 nm,was produced through the above grinding. The above-ground carbon fiberwas observed under a scanning electron microscope, micrographs of thecarbon fiber were taken, and the lengths of 100 fiber filaments of thecarbon fiber were measured by use of a vernier caliper, to therebyobtain the distribution of the lengths. The results are shown in FIG. 1.In this case, the standard deviation was found to be 0.10 μm (100 nm).In infrared analysis, absorption of light attributed to stretchingvibration of a hydroxyl group was observed at 3,600 cm⁻¹.

EXAMPLE 2

Vapor grown carbon fiber (2 g) which had undergone graphitizationtreatment in the presence of a boron compound, the carbon fiber havingan average diameter of 33 nm, an average length of 16,500 nm, and anaspect ratio of 500 and containing branched vapor grown carbon fiber inan amount of 30 mass %, ethanol (50 g), and zirconia beads (diameter ofeach bead: 1.0 mm) (200 g) were placed in an agate-made planetary mill(capacity: 300 cm³), and the carbon fiber was subjected to grindingtreatment for four hours. Subsequently, the thus-ground carbon fiber wasdried at 150° C. for three hours. Thereafter, the resultant vapor growncarbon fiber was observed under a scanning electron microscope, and thelengths of fiber filaments of the carbon fiber were measured. Inaddition, the carbon fiber sample was subjected to infrared analysis.The boron content of the carbon fiber sample was found to be 0.7 mass %.

As a result, a fine carbon fiber having an average diameter of 33 nm, anaverage length of 420 nm, an aspect ratio of 13 and d₀₀₂ of 0.337 nm,was produced through the above grinding. The above-ground carbon fiberwas observed under a scanning electron microscope, micrographs of thecarbon fiber were taken, and the lengths of 50 fiber filaments of thecarbon fiber were measured by use of a vernier caliper, to therebyobtain the distribution of the lengths. The results are shown in FIG. 2.In this case, the standard deviation was found to be 0.22 μm (220 nm).In infrared analysis, absorption of light attributed to stretchingvibration of a hydroxyl group was observed at 3,600 cm⁻¹.

COMPARATIVE EXAMPLE 1

Vapor grown carbon fiber (90 g) which had undergone graphitizationtreatment, the carbon fiber having an average diameter of 33 nm, anaverage length of 16,500 nm, and an aspect ratio of 500 and containingbranched vapor grown carbon fiber in an amount of 30 mass %, was placedin an alumina-made ball mill (capacity: 2,000 cm³), and the carbon fiberwas subjected to grinding treatment at 75 rpm for 18 hours. In thiscase, 30 alumina balls (diameter of each ball: 30 mm) were placed in theball mill for grinding of the carbon fiber. Thereafter, the resultantvapor grown carbon fiber was observed under a scanning electronmicroscope, and the lengths of fiber filaments of the carbon fiber weremeasured. In addition, the carbon fiber sample was subjected to infraredanalysis.

As a result, a carbon fiber having an average diameter of 33 nm, anaverage length of 4,980 nm, and an aspect ratio of 150 was produced. Theabove-ground carbon fiber was observed under a scanning electronmicroscope, micrographs of the carbon fiber were taken, and the lengthsof 50 fiber filaments of the carbon fiber were measured by use of avernier caliper, to thereby obtain the distribution of the lengths. Thedistribution of the fiber filament lengths is shown in FIG. 3. Thestandard deviation was found to be 3.07 μm (3,070 nm). In infraredanalysis, virtually no absorption of light attributed to stretchingvibration of a hydroxyl group was observed.

EXAMPLE 3

Each of the fired fine carbon fiber of Example 1 which had undergone wetgrinding, the graphitized fine carbon fiber of Example 2 which hadundergone wet grinding, and the graphitized carbon fiber of ComparativeExample 1 which had undergone dry grinding was mixed with a phenol resinin an amount of 40 mass %, and the viscosity (cP or mPa·s) of theresultant mixture was measured at 25° C. by use of a viscometer througha method as specified by JIS K7117. The results are shown in Table 1.

TABLE 1 Sample Viscosity (cP) Fired fine carbon fiber produced throughwet 30 grinding (Example 1): phenol resin = 40 mass %: 60 mass %Graphitized fine carbon fiber produced through 50 wet grinding (Example2): phenol resin = 40 mass %: 60 mass % Graphitized fine carbon fiberproduced through 150 dry grinding (Comparative Example 1): phenol resin= 40 mass %: 60 mass %

The viscosity of a compound prepared through mixing a phenol resin withthe fine carbon fiber produced through wet grinding (Example 1 or 2) is⅓ or less the viscosity of a compound prepared through mixing a phenolresin with the fine carbon fiber produced through dry grinding(Comparative Example 1). That is, the compound prepared from the finecarbon fiber of Example 1 or 2 exhibits improved handling.

INDUSTRIAL APPLICABILITY

(1) The carbon fiber of the present invention exhibits excellentworkability when mixed with a matrix formed of, for example, resin, thecarbon fiber is sufficiently dispersed in the resin, and the surfacesmoothness of the resultant composite material is improved.

(2) Since each fiber filament of the carbon fiber of the presentinvention has a minute depression and a hollow space to readily causeaddition reactions with hydrogen and methane, the carbon fiber issuitable as a material for sorption of a gas such as hydrogen ormethane.

(3) In the fine carbon fiber produced through the method of the presentinvention, the distribution of the lengths of fiber filaments is narrow,and variation in the lengths is small. Therefore, when the fine carbonfiber is employed as an electrically or thermally conductive filler, thequality of the resultant composite material can be enhanced.

1. A vapor grown fine carbon fiber containing branched vapor-growncarbon fiber filaments, each fiber filament of the carbon fibercomprising, in its interior, a hollow space extending along the fiberfilament, and having a multi-layer structure, an outer diameter of 2 to500 nm, and an aspect ratio of 1 to 100, wherein the fiber filamentcomprises a cut portion on its surface along the hollow space, and thestandard deviation in the distribution of the length of the fiberfilaments is 2.0 μm or less.
 2. The fine carbon fiber as claimed inclaim 1, wherein the cut portion on the surface of the fiber filamentcomprises a minute depression.
 3. The fine carbon fiber as claimed inclaim 2, wherein the minute depression is in communication with thehollow space in the interior of the fiber filament.
 4. The fine carbonfiber as claimed in any one of claims 1 through 3, wherein the fiberfilament has a functional group on a surface thereof.
 5. The fine carbonfiber as claimed in claim 4, wherein the functional group is at leastone species selected from the group consisting of a hydroxyl group, aphenolic hydroxyl group, a carboxyl group, an amino group, a quinonylgroup, and a lactone group.
 6. The fine carbon fiber as claimed in claim1, wherein the hollow space is partially closed.
 7. The fine carbonfiber as claimed in claim 1, wherein the carbon fiber comprises carbonhaving an average interlayer distance d₀₀₂ of (002) carbon layersmeasured by X-ray diffraction method of 0.342 nm or less.
 8. The finecarbon fiber as claimed in claim 1, which further comprises boron or aboron compound.
 9. The fine carbon fiber as claimed in claim 8, whereinboron is contained, in an amount of 0.01 to 5 mass %, in carbon crystalsconstituting the carbon fiber.
 10. A fine carbon fiber mixturecomprising a fine carbon fiber as recited in claim 1 in an amount of 5mass % to 80 mass % on the basis of the entirety of the carbon fibermixture.
 11. A fine carbon fiber composition comprising a fine carbonfiber as recited in claim
 1. 12. The fine carbon fiber composition asclaimed in claim 11, comprising a resin.
 13. An electrically conductivematerial comprising a fine carbon fiber as recited in claim
 1. 14. Asecondary battery comprising, as an electrode material, a fine carbonfiber as recited in claim
 1. 15. A gas occlusion material comprising afine carbon fiber as recited in claim
 1. 16. A method for producing afine carbon fiber, comprising a step of subjecting a vapor grown carbonfiber containing branched vapor grown carbon fiber filaments each fiberfilament of the carbon fiber comprising, in its interior, a hollow spaceextending along the fiber filament, and having a multi-layer structure,an outer diameter of 2 to 500 nm, and an aspect ratio of at least 10, toheat treatment and then wet-grinding the vapor-grown carbon fiber in thepresence of water and/or an organic solvent so as to generate a cutportion on the fiber filament surface along the hollow space.
 17. Themethod for producing a fine carbon fiber as claimed in claim 16, whereinthe wet-grinding is performed in the presence of a surfactant.
 18. Themethod for producing a fine carbon fiber as claimed in claim 16, whereinthe vapor-grown carbon fiber is contained at 1 to 30 mass % in wateror/and organic solvent.
 19. A method for producing a fine carbon fiber,comprising, a step of obtaining the fine carbon fiber according to claim16, and then subjecting the fiber, to which boron or a boron compound isadded or not added, to heat treatment at a temperature of 2,000° C. to3,500° C.
 20. A fine carbon fiber produced through a production methodas recited in any one of claims 16 through 19.